1. Field of the Invention:
The present invention relates to a trigger device for an electronic flash, particularly suited to daylight flashphotography for cameras with an electromagnetically driven shutter and an electric flash trigger signal.
2. Description of the Prior Arts:
Generally, in cameras connectable to flash devices or having a built-in flash device, the flash synchronization is carried out via a contact in operative engagement with the members which drive the shutter blades or the shutter curtain.
Recently, the shutter having electromagnetically driven shutter blades have been proposed and brought into practice. This kind of shutter has been disclosed, for example, in U.S. Patent Application Ser. No. 114,497 filed on Jan. 23, 1980 and U.S. Pat. Application Ser. No. 75,943 filed on Aug. 7, 1980.
In an electromagnetic shutter, the shutter blades are closed by a spring and opened by an electromagnetic force, return to the totally closed position occurs when the electromagnetic force is released.
However, with this construction it is impossible to obtain a large driving force. Furthermore, because the time during which the electromagnetic force is applied to the shutter is changed, to vary the aperture of the shutter in accordance with the brightness, it is difficult to open and close the synchronization contact mechanically. There is also a possibility that the shutter blades are not controlled up to the totally opened position depending on the duration of the electromagnetic force application.
In order to overcome this difficulty, as in the case of the system disclosed in U.S. Patent Application Ser. No. 298,532 filed on Sept. 1, 1981, an electrically produced trigger signal for the flash, said signal produced when the release of the electromagnetic force, occurs may be considered.
In the case of the shutter which is opened directly by the electromagnetic force, the blades continue the opening operation for a while due to the moment of inertia and begin closing with the strength of the spring only after the blades have no more inertia if the electromagnetic force is released before the arrival at the totally opened position, however, the shutter blades assume the closing operation immediately after the electromagnetic force for driving the shutter blades is released when the shutter blades are totally opened. Consequently, it is necessary to determine the time for applying the electromagnetic force, taking the moment of inertia into consideration. If the object field brightness is so high that the shutter blades open small, the electromagnetic force is released before the shutter blades have opened, whereby the shutter blades open inertially.
Here also, the trigger signal for the flash is a problem. Excepting the case where the shutter blades are totally opened, where the electromagnetic force is released before the shutter blades have been totally opened there is a possibility that the trigger signal for the flashlight is delivered before the blades have been opened at all. For example, if the flash is made use of as the auxiliary light for a backlighted object under daylight, there is a possibility that only the background is bright and the object to be photographed is dark.